Several processes for the solution spinning of high molecular weight polymers have been described in the prior art. The solution spinning of high molecular weight polyethylene for example, has been described in U.S. Pat. Nos. 4,551,296; 4,344,908 and 4,430,383, all of which are hereby incorporated by reference. The solution spinning of high molecular weight polyvinyl alcohol was described in U.S. Pat. No. 4,440,711. In each of these processes a homogeneous solution of the high molecular weight polymer in the solvent is required.
The preparation of homogeneous solutions of high molecular weight polymers is not a trivial art. U.S. Pat. No. 4,344,908 describes only a batchwise (discontinuous method). U.S. Pat. No. 4,430,383 is totally silent on a method and means of preparing the polymer solution. Some of the difficulties of preparing a high molecular weight polymer solution have recently been discussed in Japan, Kokai patent publication No. 52326-1985 as follows:
"Now, in the production of the above mentioned gel fiber, the extrusion spinning apparatus similar to those used in the common melt spinning is used; however, in this case, the greatest difficulty lies in the preparation of homogeneous spinning solution of required concentration. Thus, in order to obtain the homogeneous spinning solution, the synthetic polymer and the solvent are placed in a mill at a temperature near the decomposition temperature of the synthetic polymer and this has to be stirred and mixed for a long time. However, even with this, homogeneous dissolving of the synthetic polymer into the solvent cannot be achieved adequately in many cases. Particularly, in order to obtain the high strength, high modulus fibers, one needs the synthetic polymer of ultrahigh molecular weight and, in this case, dispersion and dissolving of the synthetic polymer into the solvent is even more unhomogeneous. Also, at the time of dissolving, the solution viscosity rises and mixing of gas bubbles cannot be avoided in the stirred mixing process. The lumps of molecules which remain undissolved, mixing of gas bubbles, and the nonhomogeneity of the solution tie to the instability in yarn making in the spinning process and, in some cases, they make the yarn making impossible. Also, the resulting final fibers have inferior quality."
"As to the past technology for resolving these shortcomings, there is the method of spinning in which a plural number of dissolving tanks are installed in the solution spinning process and powerful stirring in the first dissolving, second dissolving--and long dissolving time are used to achieve the homogeneous dissolving; then, while keeping the solution at a high temperature, the solution is fed to a screw type extruder for spinning. In this dissolving method, homogeneous solution of certain extent of homogeneity can be obtained but the dissolving takes a long time. Particularly, in order to obtain the high strength, high modulus, high toughness fiber, it is necessary to use the synthetic polymer of ultrahigh molecular weight but, in the case of the ultrahigh molecular weight polymers, the dissolving needs to be done at high temperature for a long time; as a consequence, a large drop in the molecular weight of the synthetic polymer is induced. Particularly in the dissolving tank method, homogeneous dissolving is achieved by powerful stirred mixing and, consequently, mixing of fine gas bubbles into the solution cannot be avoided. Thus, the dissolving tank method has numerous shortcomings, such as the need for installing multiple stage dissolving tanks, long time needed for the dissolving, severe drop in the molecular weight of the synthetic polymer, and easy mixing of gas bubble. Furthermore, when the dissolved solution made by such conventional method is fed to the spinning process, stable spinning for long time cannot be achieved, and the resulting fibers have inferior quality and so the process is not adequate for industrial production."
Kokai patent publication No. 52326-1985 described a complex procedure to avoid those problems. In their method the polymer is first dissolved in a solvent and then is cooled to make gel particles containing the solvent. The gel particles are dispersed or dissolved in the same or different solvent, and the dispersion or solution is fed to the spinning extrusion apparatus.
Other complex solution preparation procedures are described in Japan, Kokai patent publication No. 34614-1985 and Japan, Kokai patent application No. 223307-1984. In a related vein U.S. Pat. No. 4,545,950 describes a complex process involving melt kneading of a high molecular weight polymer with a wax, followed by melt extrusion.
By extruding device is meant herein a spinnerette, or extruder for film or tape or other extruders which form articles from solutions of high molecular weight polymers.
The method of continuously preparing solutions of high molecular weight polymers described in U.S. Pat. Nos. 4,551,296 and 4,440,711, hereby incorporated by reference, is somewhat less complex. This method consists of:
a. Forming a slurry of the high molecular weight polymer in a solvent. PA1 b. Transferring that slurry to an intensive mixing device in which the residence time, agitator speed, and temperature is sufficient to convert the slurry to a solution. PA1 c. Transferring the solution of this high polymer to an extrusion device to deliver polymer solution at a reasonably high pressure to a metering (gear) pump. PA1 a. Forming a slurry of particles of the high molecular weight polymer in a solvent for the polymer. PA1 b. Transferring that slurry to a pumping device and thence, under sufficient pressure to insure positive conveyance, to a screw extruder. PA1 c. Maintaining the slurry in the screw extruder for sufficient residence time and at a sufficient temperature that a homogeneous solution of the polymer is formed, and then this solution is conveyed by the screw extruder under pressure to a metering device.